1. Field of the Invention
The present invention relates to a coupling structure of a waveguide and an applicator, and more particularly, to a coupling structure of a waveguide and an applicator which is capable of controlling propagation of an electromagnetic wave generated by an electromagnetic wave generator to be transmitted to an applicator in one direction, and of maintaining a stable operation even though the state of a load is varied.
2. Description of the Background Art
A system, in which the electromagnetic wave generated by an electromagnetic wave generator such as a magnetron is transmitted through a waveguide to a load inside an applicator, is used in various fields such as a microwave oven, an electrodeless lamp or a heating instrument.
Generally, this type of applicator includes a waveguide type or a cavity type. The cavity type applicator consists of a resonant type and a non-resonant type, and the waveguide type applicator consists of a cylindrical type and a rectangular type according to the shape of its cross-section. The waveguide type applicator utilizes either TEmn, or TMmn modes of electromagnetic field distribution inside the waveguide. Here, xe2x80x98mxe2x80x99 and xe2x80x98nxe2x80x99 are natural numbers inclusive of xe2x80x980xe2x80x99.
In general, the mode with the smallest cutoff frequency for a given dimension of a waveguide, or the mode formed by an electromagnetic wave of the lowest frequency that can propagate in a waveguide is called the dominant mode, and in this respect, in case of the cylindrical type waveguide, its dominant mode is TE11, while that of the rectangular type waveguide is TE10.
The resonant type cavity is classified into TEmnp and TMnmp types depending on the electromagnetic field distribution mode inside the cavity, and if a cavity is capable of supporting plural modes therein at the same time, it is called a multi-mode cavity. A typical example of the multi-mode cavity is that of a microwave oven.
In most cases, the load inside the applicator is in a solid state or in a liquid state, but gas also can be the load, for example, in case of a plasma generator. The load may have various shapes, and may be fixed or moving.
Conventionally, it has been hard to maintain a stable operation due to the change of the load state.
For example, referring to the electrodeless lamp, it is very difficult to simultaneously satisfy conditions for igniting a lamp bulb and for maintaining stable operation of the lamp. The impedance of the lamp bulb or resonator including the lamp bulb varies significantly depending on the state of the lamp bulb.
That is, since the impedance of the bulb in different states, for example, when it is cold with no discharge, when it starts to discharge, or when the lamp is fully activated and maintain a steady state, when matching is made for a specific state, other states are considerably mismatched.
Therefore, even if the lamp bulb is ignited and initial discharge is activated, the lamp bulb is likely to be turned off while it is proceeding to a stable state, or even though the lamp bulb reaches the stable state, since the impedance matching of the bulb to the electromagnetic wave becomes poor, the overall luminous efficiency of the system is significantly degraded.
In order to avoid the loss of efficiency, generally, impedance matching of the system is set at a value at which the lamp bulb is in a stable state of operation. However, in this case, since the matching in the initial state of the lamp bulb is not properly made, the electromagnetic wave applied to the resonator is mostly reflected back to the magnetron. Due to the reflected electromagnetic wave, the electric field inside the resonator is not strong enough to ignite the lamp bulb in it, and thus it is difficult to ignite the lamp bulb. In addition, the magnetron may operate unstably or oscillate abnormally, or the temperature of the magnetron may rise, so that the durability of the magnetron is decreased significantly. In this case, in order to ignite the lamp bulb, resonators of complicated shapes are used, or a device that helps to ignite the lamp bulb is added in the resonator. However, its expense is inevitably increased and its structure becomes complicated. Also, with these methods, the problem of the abnormal operation or the short life of the magnetron is not solved.
Meanwhile, when the characteristic impedance of the load system to the electromagnetic wave, that is, the combined impedance of the applicator and the bulb inside it, and that of the waveguide transmission line do not agree, electromagnetic waves reflect back from the applicator. In this case, as the reflected energy returns back to the electromagnetic wave generator, it has an adverse effect on the electromagnetic wave generator by disturbing stable operation of it, or is absorbed as heat in the electromagnetic wave generator, thereby shortening its life, or even destroying it. Therefore, a tuner or a circulator is generally used to protect the generator and to ensure proper matching.
FIG. 1 is a schematic view of a structure of a waveguide system in accordance with conventional art.
The tuner 2 controls the characteristic impedance of the waveguide transmission line. A directional coupler 4 of FIG. 1 extracts a predetermined fraction of the electromagnetic wave that proceeds toward the load 6 or turns back after reflecting from the load in the applicator 5. In order to maintain a good matching state in the transmission line, a wattmeter 3 is connected to the directional coupler 4 and the tuner 2 is adjusted so that the reflected wave is minimized.
Reference numbers 1 and 2 in FIG. 1 show a magnetron and a waveguide, respectively.
However, the conventional art has disadvantages in that the tuner 2 needs to be adjusted according to the state of the load in order to maintain the matching satisfactorily. Especially, in case that the characteristics of the load 6 varies during its operation, the tuner 2 needs to be continuously adjusted. Moreover, if the impedance of the load 6 changes irregularly or drastically, it is difficult to maintain a favorable matching state.
In addition, using the tuner 2, the directional coupler 4, and the wattmeter 3 or circulator causes an increase in expense and the overall size of the system to be enlarged and complicated.
Therefore, a new waveguide structure for overcoming the shortcomings of the conventional art is required.
Therefore, an object of the present invention is to solve the problem in a system in which an electromagnetic wave is transmitted from an electromagnetic wave generator to an applicator, and in which energy reflected due to the variation of the load characteristics is returned to the electromagnetic wave generator, degrading the characteristics of the electromagnetic wave generator.
Another object of the present invention is to eliminate the inconvenience of adjusting a tuner in a waveguide structure in response to occasional variation of the load characteristics.
To achieve these and other advantages, and in accordance with the purposed of the present invention, as embodied and broadly described herein, there is provided a coupling structure of a waveguide, and an applicator including: an electromagnetic wave generator, a waveguide for transmitting an electromagnetic wave generated by the electromagnetic wave generator; and an applicator for receiving the electromagnetic wave through the waveguide and applying it to a lamp bulb, wherein the walls of the waveguide and the applicator are partially or wholly shared, said wall having slots formed thereon, and the length of the waveguide is equal to an integer times one-half of the wave length of the electromagnetic wave that is guided through the waveguide.
At least two slots are formed at certain intervals on the wall held in common by the waveguide and the applicator, causing the electromagnetic wave reflecting from the applicator to not return back to the electromagnetic wave generator when it is directed to the waveguide.
The interval between center points of the slots is equal to approximately one fourth of the wave length of the electromagnetic wave transmitted in the waveguide. The width of the slot is preferably more than three times the thickness of the wall where the slots are installed.
The waveguide is rolled in a cylindrical form centering around the axis of the resonator, so that the propagation trajectory of the electromagnetic wave within the waveguide forms a concentric circle or a concentric circular arc to the cross section of the resonator.
An electromagnetic wave absorbing unit is additionally provided at an end portion of the waveguide in the propagation direction of the electromagnetic wave, so as to absorb the electromagnetic wave still proceeding inside the waveguide without being coupled to the applicator, and the electromagnetic wave returning to the waveguide is being reflected from the applicator and proceeding in its initial direction (that is, the opposite direction to the electromagnetic wave generator). As for the absorbing unit, carbon, graphite or water may be used therefor.
Referring to the cross section shape of the applicator, a circular or an oval shape is appropriate, and as for the cross section of waveguide, a semicircular, a circular, or an oval shape is appropriate.
To achieve the above objects, there is also provided an electrodeless lamp including: an electromagnetic wave generator; a waveguide guiding the electromagnetic wave generated by the electromagnetic wave generator; a resonator for receiving the electromagnetic wave from the waveguide and applying it to a lamp bulb; and an electrodeless bulb within the resonator, wherein the wall of the waveguide and the applicator are partially or wholly held in common, on which slots are formed, so that the electromagnetic wave does not return back toward the electromagnetic wave generator when it is reflected from the resonator, and the length of the waveguide is equal to an integer times one-half of the wave length of the electromagnetic wave guided within the waveguide.